Conventionally, heat pump air conditioners and freezers that perform heat exchange with air are provided. A heat pump for use in, for example, an air conditioner, absorbs heat from cold air during winter, and thus its heat exchanger is frosted. In the case of a heat pump for use in a freezer, its heat exchanger is cooled to a temperature below the freezing point in order to generate an intended low temperature, as a result of which the heat exchanger is frosted. The frost layer has a low thermal conductivity and thus serves as a heat insulator, causing a reduction in the operational efficiency of the heat pump. For this reason, when frost is formed, it is necessary to remove the frost.
In a conventional heat pump, a defrost operation is performed such that upon detection of the degree of frosting based on the refrigerant pressure or the like, the operation is temporarily stopped so as to reverse the refrigeration cycle to perform thawing with hot gas. Alternatively, there is a conventional heat pump in which its evaporator is caused to function as a condenser by counter-rotating the refrigerant so as to perform thawing. Patent Document 1 discloses a refrigeration cycle apparatus that performs a defrost operation by switching a direction of flow of the refrigerant such that the function of the heat exchanger is reversed by a four-way switching valve.
However, in the case where the refrigerant is counter-rotated during a defrost operation of the heat pump as in Patent Document 1, it is necessary to intermittently stop its heat exchange operation, and thus a problem arises in that the heat pump cannot be operated continuously. Also, because it is not possible to absorb heat for use to perform defrosting from the other side of the heat exchange operation (for example, it is not possible to absorb heat of indoor air by performing a defrost operation during heating operation), the amount of heat for defrosting is dependent exclusively on the pump work. At this time, the COP (coefficient of performance) is 1, and thus it is a cause of reduction of the COP of the heat pump as a whole.
Frosting is of significant value in terms of acquiring heat of solidification although it is problematic in that it causes a reduction in thermal conductivity. During heating, a heat pump uses, in addition to the sensible heat of air and moisture, the heat of condensation and heat of solidification (both of which are latent heat) of moisture. A test conducted by the present inventors revealed that the latent heat accounts for up to 40% of the total amount of heat exchanged (0 to 40% at a relative humidity of 50 to 80%).
From this, a situation is conceivable in which if frosting does not occur at all, the heat obtained from the heat pump also becomes insufficient. Accordingly, if defrosting can be performed mechanically (physically) instead of thawing the frost by heat, it may be possible to utilize heat of solidification to the maximum extent possible without causing an energy loss. However, as widely known, the crystals of solidified ice are hard, and it is not easy to remove the crystals mechanically.
In view of the above, the present inventors developed a heat exchanger that can mechanically remove the frost formed on the heat exchanger with ease (Patent Document 2). In the heat exchanger according to Patent Document 2, very fine protruding portions and recess portions are formed on the surface of a fin used in the heat exchanger. With this configuration, frost crystals grow vertically on the flat surface portions on top of the protruding portions, which creates gaps in the recessed portions. As a result, frost crystals having a comb-like shape as a whole are formed on the fin. The frost crystals having such a shape are structurally weak, and thus can be easily removed by mechanical removal means using a brush, a scraper, or the like. Therefore, according to Patent Document 2, the heat exchanger can operate continuously for a long period of time while utilizing heat of solidification.